Medical imaging apparatuses with spectrally resolved photon counting apply radiation detectors that can count the number of (typically X-ray or gamma) photons hitting the pixels of the detector and that can classify them according to their energy. This is usually achieved by converting the photons into charge, current and/or voltage pulses that can readily be counted, wherein the pulse height or integral indicates the photon energy.
One problem such detectors have to cope with is the pile-up or overlap of subsequent pulses in case of high count rates. Another problem is the splitting of generated charges between neighboring pixels. Both effects lead to erroneous interpretations in terms of counting rates and/or photon energy.
The U.S. Pat. No. 4,658,216 discloses an approach to deal with the pile-up problem by converting current pulses into two pulses of different time constants, wherein the shorter pulse is processed by a pile-up rejector such that evaluation of the longer pulses is inhibited in case of pile-up.
The WO 2008/146230 A2 discloses an apparatus in which the heights of pulses generated by incoming X ray photons are compared to thresholds to count them in different energy bins. In parallel, the maxima of the pulses are detected. If the temporal distance between maxima is within a short time window, this is taken as an indication of pile-up, and the associated contributions to the energy bins are disregarded.
The EP 0 396 464 A2 discloses a nuclear spectroscopy method in which pulses generated by gamma rays are continuously converted into digital samples. A pulse pile-up is assumed if the number of digital samples exceeds a given maximum count.
Furthermore, a theoretical analysis about how pile-up affects the spectrum in a gamma ray detector has been described in literature (CHAPLIN V.: “Analytical Modeling of pulse-pileup distortion using the true pulse shape; applications to Fermi-GBM”, NUCL. INST. AND METH. IN PHYSICS RESEARCH A, vol. 717, 2013-04-06, pages 21-36)).
The U.S. Pat. No. 4,634,863 A discloses a pulse processing-circuit that processes the trailing portion of a pulse differently from the rising portion, allowing for a better separation of ensuing pulses.